PSA (Polymer Sustained Alignment)-type liquid crystal display devices have a structure in which a polymer structure is formed in a cell in order to control the pretilt angle of liquid crystal molecules, and have been progressively developed as liquid crystal display devices in view of their rapid response and high contrast.
A PSA-type liquid crystal display device is manufactured by injecting a polymerizable compound-containing liquid crystal composition containing a polymerizable compound and a liquid crystal compound between substrates, and polymerizing the polymerizable compound under a condition where liquid crystal molecules are aligned by applying a voltage, thereby fixing the alignment of the liquid crystal molecules. In the PSA-type liquid crystal display device, impurities and a change in alignment of the liquid crystal molecules (change in pretilt angle) are known as causes for image sticking as a display defect.
A cause for a change in pretilt angle is that in a configuration of a liquid crystal display device using a soft polymer which is a cured product of a polymerizable compound, a polymer structure is changed when the same pattern is continuously displayed for a long time, resulting in a change in pretilt angle. Therefore, a polymerizable compound which forms a polymer having a rigid structure with no change in the polymer structure is required.
In order to prevent image sticking by improving the rigidity of a polymer, it has been investigated to constitute a display device using a polymerizable compound having a structure of 1,4-phenylene group or the like, which contains only a ring structure and a polymerizable functional group (refer to Patent Literature 1), or constitute a display device using a polymerizable compound having a biaryl structure (refer to Patent Literature 2). However, such polymerizable compounds have low compatibility with liquid crystal compounds and thus, when a liquid crystal composition containing a polymerizable compound is prepared, the problems such as precipitation of the polymerizable compound are produced, leading to the need for improvement in compatibility with the liquid crystal composition.
Also, in order to prevent image sticking by improving the rigidity of a polymer, it has been proposed to constitute a display device using a mixed liquid crystal composition containing a bifunctional polymerizable compound and a tri- or higher-functional polymerizable compound, such as dipentaerythritol pentaacrylate, dipentaerythritol hexaacrylate, or the like (refer to Patent Literature 3). However, dipentaerythritol pentaacrylate and dipentaerythritol hexaacrylate have no ring structure in the molecules thereof and thus have weak affinity for liquid crystal compounds and weak force to regulate alignment, thereby causing the problem of failing to achieve satisfactory alignment stability. In addition, polymerization of these polymerizable compounds requires a polymerization initiator to be added, and if a polymerization initiator is not added, the polymerizable compounds remain after polymerization.
In addition, various combinations of liquid crystal compositions and polymerizable compounds (refer to Patent Literature 4) are disclosed as methods for improving switching speeds. However, a relation between the pretilt angle and switching speed is generally known, and a significant improvement effect has not been confirmed. Since liquid crystal compounds containing an alkenyl group and chlorine atom are used, a defective display is highly likely to be caused, resulting in impracticality.
Therefore, there has been demand for a polymerizable compound-containing liquid crystal composition which simultaneously resolves the problems of preventing the occurrence of precipitation in a wide temperature range and the occurrence of image sticking, and achieving a fast switching, etc., and a liquid crystal display device using the liquid crystal composition.